1. Field of the Invention
This invention is directed to an improvement in an extruder head utilized to form concentric tubes wherein an outer tubular extrusion or sheath is formed about an inner tube or cable. This invention is particularly directed to an apparatus for forming extruded tubes or sheaths of highly viscous synthetic material, e.g., polyvinyl chloride, about an interior inner tube or cable.
2. Discussion of the Prior Art
Extruder heads in which a stream of synthetic plastics is side fed at right angles or obliquely to the extrusion axis for the purpose of extruding a plastic outer tube or sheath about an inner tube or cable conducted axially through the extruder head are already known in the art. Double extruder heads for extruding two concentrically interposed tubes are also known in which the synthetic plastics for the outer tube is axially conducted, whereas the plastics material for forming the inner tube is cross fed from a transverse or oblique feed head into the interior of the plastics stream for forming the outer tube.
In these known types of cross or obliquely fed extruder heads a difficulty which arises in that the laterally entering plastic stream impinges upon the inner tube of the extruder head and must flow around this tube. In doing a considerable back pressure builds up. Consequently, these known types of cross or oblique feed extruder heads are suitable only for extruding synthetic plastics materials that are not very viscous in their plastically flowable state. When extruding highly viscous plastics materials, such as hard PVC, the back pressures often builds up to several hundred atmospheres. These are pressures which in practice cannot be adequately controlled so that even when using strong flanges the flange bolts yield and tear out.
It is an object of this invention, therefore, to provide an extruder head which is suitable for extruding an outer tube or sheath about an inner tube or cable which is particularly suitable for those plastics material which are highly viscous under the usual extrusion conditions. It is an object of this invention, additionally, to provide an extrusion head wherein the plastic material can still be fed to the extruder head at an angle thereby rendering it unnecessary to provide a radically different structure from those extrusion heads heretofore employed. It is another object of this invention to provide an extrusion head for extruding an outer tube or sheath about an inner tube or cable where the plastic material fed for the formation of the outer tube or sheath is fed at an angle acute to the extruder head, i.e., at an angle generally acute with respect to the flow of molten plastic material towards the exit of the extruder head.